It is known that air pollution problems each day become more and more serious and, are among them, those which are associated with air pollution by the exhaust gases of motor vehicles.
It is known that one of the methods which seems to be very efficient to the end of the solution of these latter problems is the installation of a catalytic muffler in the exhaust duct of an engine, so that the exhaust gases are passed through said muffler before emerging into the outside atmosphere.
The exhaust gases reach the muffler while they contain unburned fractions of several kinds, but they also contain an amount of oxygen which is such as to allow the combustion of these unburned fractions; in the case in which the aforementioned amount of oxygen is not yet contained in the exhaust gases emerging from a cylinder, oxygen is injected into the exhaust stream prior to entering the muffler. Obviously, in a catalytic muffler, the presence of a catalyst is responsible for encouraging the combustion of the unburned fractions by the action of oxygen.
At the same time, a number of problems must be solved when providing a catalytic muffler; among these, the following are considered essential:
(1) Utilization of the catalyst mass so as to have a maximum efficiency without impairing the engine performance. PA1 (2) Maintenance of this efficiency in time. PA1 (3) Technical manufacture of the container in consideration of its service life. PA1 (4) Matching the shape and the size of the muffler with the space requirements in the vehicle.
In connection with the last of the problems enumerated above, the muffler according to the present invention has a flattened shape so that it can be positioned under the vehicle floor without reducing (or reducing to a minimum only) the roominess of the vehicle while maintaining an appropriate distance from the ground. For a vehicle having a front engine, since the temperature decreases along the exhaust pipe towards the outlet end, this approach permits the installation of the muffler at the most appropriate distance from the engine as regards the optimum temperature required for the operation of the catalyst material (see problem No. 1 above).
In order not to impair the efficiency of the engine, (see problem No. 1) it is required, on the other hand, that the pressure drop which the exhaust gas undergoes while flowing through the muffler be small, so that it is imperative both to limit as far as practicable the thickness of the catalyst mass through which the exhaust gases flow and the flow speed: both of these requirements have been fulfilled by providing that the direction of flow through the catalyst mass of the flat muffler (positioned horizontally beneath the vehicle) is vertical.
In further regard to the first of the problems enumerated above, considerations on the physical phenomena which take place in the exhaust gases as they flow through the catalyst mass, have suggested to direct the vertical flow downwards: as a matter of fact in the cold-engine transitional stage, that is, when the exhaust gases are not too hot and on the other hand, also the catalyst mass is cold and draws heat from the gases (a certain temperature should be attained for the catalyst material to become efficient), the gases are so cooled that condensation of the reaction water is experienced. The water, due to the action of the pressure differential upstream and downstream of the catalyst mass, to which the effect of the gravity pull is added in the case of a downward flow, is driven out of the catalyst mass, with a considerable advantage as regards the heating time of the mass inasmuch as, subsequently, when 100.degree. C is locally exceeded, these water droplets are no longer present: these, due to their tendency to evaporate, would draw heat from the exhaust gases thus delaying the catalyst warming up. The vertical flow has then been directed downwards also in connection with the second of the problems above enumerated. In the very frequent case of granular catalysts, the service life could be impaired, in fact, by known friction phenomena, that is the crushing of the granules in their mutual contact areas as a result of the relative motion of any granule with respect to the others. These relative motions can both be induced by the accelerations due to the vibrations to which the muffler is subjected for its being connected to the engine, and by the pulsations of the exhaust gas flow which is passed through the granulated mass.
A comprehensive investigation of the phenomenon has shown that the relative intergranular motions can be prevented only if the pressure differential of the gas as due to its flowing through an individual layer of granules, acts upon any granule in the same direction as the gravity pull and if such a pressure differential exceeds a certain value at which the force applied to a granule exceeds by a certain amount the force acting on the granule due to the vibrations and thus to the cyclically variable accelerations to which the muffler (and thus a granule, supposed integral therewith) is subjected. It has also been ascertained that the attainment of such optimum conditions, with the muffler configuration as suggested herein, can be obtained irrespective of the conditions of use of the engine. On the one hand due to the light weight off the granules and, on the other hand since, at low running speeds of the engine, the gas rates of flow and thus the pressure differentials, are small but also the magnitude of thee vibrations is small, whereas, at a high rate of revolution of the engine, the vibrations are more intense, but also greater are the gas rates of flow and thus also the pressure differentials.
On account of the flattened shape indicated above and thus the reduced thickness of the catalyst layer through which the gas is required to flow, a considerable difficulty, however, is experienced to the end of the first problem, that is, the optimum utilization of the catalyst mass in consideration of the noticeable area of the cross-section which is perpendicular to the flow; the utilization will be at an optimum only if the flow speed of the gas through the layer is equal at all the points of the normal cross-section aforesaid. This can be obtained only if the pressure differential upstream and downstream of the layer is equal for all the points of the cross-section concerned. Space requirements (see problem No. 4) prevent the adoption of the most obvious solution, that is, to provide two considerable volumes, one (having the function of a conveying device) immediately upstream, and the other (acting as manifold) downstream of the layer, with the pressure being constant at any point of the two volumes since the gas flow speed is extremely low.